I got curious, so I tested out two scenarios.
One had several instances of iZotope Iris, which is a fairly CPU-heavy VST3 synth. The other had several instances of Kontakt, a VST2 synth. All were fed the same dense MIDI tracks.
With the Iris project, the CPU drain increased considerably when it was playing, and went down when it was not. With the Kontakt project, the CPU power stayed constant regardless of whether the instances of Kontakt were playing or not.
This would seem to indicate that Sonar can suspend power with at least one VST3 soft synth.